The present invention relates to coal fired power plants and processes wherein pollution resulting from use of relatively low-value coal is reduced. More particularly, the invention relates to reduction of sulfur dioxide and nitrogen oxide emissions while producing useful by-products of the residue.
A typical coal fired coal power plant is equipped with a lime/limestone fuel gas desulfurization unit for the removal of sulfur dioxide from the flue gas by reacting the sulfur dioxide with lime or limestone to produce either calcium sulfite, calcium bisulfite, calcium sulfate, or a mixture of all three, collectively called gypsum. This gypsum which is typically collected in a gypsum pond is unsuitable for reuse and normally stored in a pile of fine particulate material. The conventional desulfurization unit does not adequately reduce nitrogen oxides in the flue gas. Additionally, the typical coal fired power plant has precipitators or the like to collect ash which also accumulates. Also, the typical coal fired power plant produces a bottom ash or slog which accumulates. The accumulating ashes and gypsum pose a long term potential threat to the ground water supply. Emission of NO.sub.x in the flue gas contributes to air pollution which threatens both animal and plant life.
Various approaches have been taken to find a way to reduce NO.sub.x levels passing through flue gas desulfurization units. U.S. Pat. No. 4,167,578, issued to Mehta, et al., describes a process of contacting a mixture of gases with a slurry of lime/limestone and an iron compound. The iron compound functions as a catalyst in the reaction between the nitrogen oxides in the gas mixture and sulfites and bisulfites formed when the sulfur dioxide in the mixture reacts with the lime/limestone. Ammonia is eventually recovered through the use of this process, but the final solids, CaSO.sub.4 and CaSO.sub.3, are simply discharged. U.S. Pat. No. 4,009,244, issued to Atsukawa, et al., discloses a process for removing oxides of nitrogen and sulfur from exhaust gases which includes using an alkali metal or alkaline earth metal iodide. Nitrogen oxide becomes nitrogen gas and sulfur oxides become gypsum.
Utilizing waste gypsum to produce a useful product has been previously suggested, for example, see U.S. Pat. Nos. 3,729,55, 4,162,170, and 4,247,518. One suggested process is to convert gypsum into sulfuric acid and lime or cement, including the utilization of coal to reduce the calcium sulfate within a fluidized bed reactor. The prior art processes for converting gypsum to sulfuric acid and lime or cement, including those employing fluidized bed reactors, have heretofore been uneconomical because of energy, labor and capital costs associated with the processes considered. One commercially valuable process for the conversion of gypsum into useful products is disclosed in U.S. Pat. No. 4,503,018 issued to Gardner et al., which is incorporated herein by reference in its entirety. The Gardner et al. process yields a sulfur-containing gas stream resulting from the thermal decomposition of the gypsum. More specifically, the Gardner et al. process involves charging a pelletized mixture of carbonaceous material and gypsum or phosphogypsum to a travelling grate where the mixture is dried and heated to produce a gaseous effluent containing sulfur dioxide and/or sulfur. After the pellets have undergone thermal decomposition, the lime residue may be sold or used in conventional applications. An improvement on the Gardner et al. process is disclosed in U.S. Ser. No. 927,439, filed Nov. 6, 1986, which is incorporated herein by reference in its entirety. The '439 application describes the addition of pyritic materials to the feed mix for desulfurization of phosphogypsum.
A process which combines coal gasification and gypsum desulfurization is disclosed in U.S. Pat. No. 4,744,969 to Marten et al. which is incorporated herein by reference in its entirety. The Marten et al. patent describes a process for the co-conversion of phosphogypsum and high-sulfur, low-BTU coal into sulfur-free and sulfur-rich gaseous streams, and usable aggregate.
While the Gardner et al. process and the Marten et al. process are valuable processes for converting gypsum into usable products, a need exists for a process which will reduce limestone consumption in the flue gas desulfurization step, reduce aggregate production, reduce NO.sub.x levels, and result in the recovery of essentially all of the sulfur as elemental sulfur, the most desirable by-product, using less energy and capital.